1. Field of the Invention
The present invention relates to a rare earth-iron based resin bonded magnet, and more particularly to an anisotropic rare earth-iron based resin bonded magnet with high magnetic properties that will satisfy the following conditions: when coercivity HcJ at a room temperature is approximately 1 MA/m, a squareness (achieved by calculation through Hk/HcJ) at a room temperature is Hkc/HcJRT, and a squareness at a temperature of 100° C. is Hk/Hc100, Expression Hk/HcJRT<Hk/HcJ100 is obtainable. Here, Hk is a magnetic field in a demagnetization curve corresponding to remanence. Mr, 90% magnetization. In this anisotropic rare earth-iron based resin bonded magnet, squareness deterioration based on a demagnetization curve at a high temperature can be avoided, and the maximum energy product (BH)max can be 170 kJ/m3 or more.
2. Description of the Related Art
Material types for rare earth-iron based magnet such as Nd2Fe14B base, αFe/Nd2Fe14B base and Fe3B/Nd2Fe14B base that are obtainable through rapid solidification, for example, a melt spinning method, are limited to a thin strip such as a ribbon, or powder obtained by milling the thin strip. Accordingly, for obtaining a bulked magnet applicable to a compact rotary machine, there will be necessary to conduct material transformation, that is, solidifying the thin strip or the powder into specific bulks with some measures. A primary measures to solidify the powder by means of powder metallurgy is pressureless sintering. However, it is not easy to apply the pressureless sintering to magnetic materials while maintaining their magnetic properties in a metastable condition. Based on the above, the thin strip or the powder has been solidified into specific bulks through binding materials such as epoxy resin, being able to obtain so-called resin bonded magnets.
For example, in 1985, R. W. Lee et al. reported that an isotropic Nd2Fe14B based bonded magnet with a (BH)max of 72 kJ/m3 is obtainable in such a manner that a thin strip with a (BH)max of 111 kJ/m3 is solidified with resin (see Non-Patent Document 1).
In 1986, the present inventors have proved through the Non-Patent Document 1 that an annular isotropic Nd2Fe14B magnet with a (BH)max of up to 72 kJ/m3 where the thin strip is solidified with epoxy resin is practicable to compact rotary machines. Further, for example, in 1990, G. X. Huang et al. have proved practicability of an isotropic resin bonded magnet to compact rotary machines (see Non-Patent Document 2), and in the 1990's such a isotropic resin bonded magnet has been widely become known as an annular magnet for a high-performance compact rotor machine applicable to an electromagnetic driving device in electric and electronic equipment such as OA (office automation), AV (audio and visual), PC (personal computer), PC peripheral devices, and telecommunication equipment.
On the other hand, starting from the 1980's, extensive researches on magnetic materials in a melt spinning method have been conducted. Accordingly, Nd2Fe14B based materials, Sm2Fe17N3 based materials, or nanocomposite materials through exchange coupling with αFe based or Fe3B based materials based on the forenamed materials (Nd2Fe14B based and Sm2Fe17N3 based materials) have become publicly known. Further, in addition to diversified alloy compositions or materials where the structure of the alloy compositions is subjected to fine-control, magnetic materials in different shapes obtainable by a rapid solidification method other than the melt spinning method became also known in recent (see for example, Non-Patent Documents 3 and 4). Also, Davies et al. reported magnetic materials where a (BH)max is reachable up to 220 kJ/m3 even though the magnetic materials are isotropic (see Non-Patent Document 5). However, it is speculated that the (BH)max of industrial applicable strips through the rapid solidification method is up to 134 kJ/m3, and the (BH)max of an isotropic resin bonded magnet where the stripes are solidified with resin at 0.8 to 1.0 GPa can be estimated approximately up to 80 kJ/m3.
Regardless of the above, considering electromagnetic driving devices such as relatively compact rotary machines to which the present invention relates, along with the high performability of electrical and electric equipments, demands for further miniaturization, high-output and high efficiency have never been ceased. Thus, it is obvious that just improving the magnetic properties of magnetically isotropic strips through the rapid solidification method is no longer enough for catching up with the enhancing performance of electric and electronic equipment. Accordingly, necessity has been further focused on a magnet generating static magnetic fields in which to fit the most preferable magnetic circuits for the iron core of the rotary machines (preferably, magnets that generate further strong static magnetic fields per unit volume).
Here, considering Sm—Co based magnetic materials applied for a rare-earth magnet, it is possible to obtain high coercivity (HcJ) even though ingots have been milled. However, the application of Co has problems in its stable supply due to a fragile resource balance and so on. It would be thus not suitable to apply Co as general-purpose industrial materials. On the other hand, rare earth-iron based magnetic materials that are mostly based on Fe as well as rare-earth elements such as Nd, Pr and Sm are advantageous in stable resource supplies of a resource balance. However, only a limited HcJ is obtainable even if the ingots of Nd2Fe14B based alloy or sintering magnets are milled. Accordingly, for producing anisotropic Nd2Fe14B based magnetic materials, researches where melt spinning materials are applied as starting materials were advanced.
In 1989, Tokunaga obtained an anisotropic magnet with a (BH)max of 127 kJ/m3 in such a manner as that a bulk where Nd14Fe80-XB6GaX (X=0.4 to 0.5) is subjected to hot upsetting (die-upset) is milled so as to form anisotropic Nd2Fe14B based magnetic materials where HcJ=1.52 MA/m, and the magnetic materials are then solidified with resin (see Non-Patent Document 6). Also, in 1991, H. Sakamoto et al. obtained anisotropic Nd2Fe14B based magnetic materials where HcJ=1.30 MA/m in such a manner as that Nd14Fe79.8B5.2Cu1 is subjected to hot rolling (see Non-Patent Document 7). Accordingly, high HcJ (coercive) magnetic materials become publicly available while hot processing treatments are improved with addition of Ga and Cu, and the refinement of an Nd2Fe14B crystal particle size is further advanced.
In 1991, V. Panchanathan et al. obtained a resin bonded magnet with a (BH)max of 150 kJ/m3 through a hot mill method, specifically as that the invasion of hydrogen is made from a grain boundary so as to make a bulk collapsed as Nd2Fe14BHX, and then HD (hydrogen decrepitation) —Nd2Fe14B magnetic materials that have been dehydrogenated by vacuum heating are extracted. Finally, the magnetic materials are solidified by resin (see Non-Patent Document 8). In 2001, through the same method, Iriyama obtained a modified anisotropic magnet with a (BH)max of 177 kJ/m3 by making Nd0.137Fe0.735CO0.067B0055Ga0.006 into magnetic materials and then solidified with resin (see Non-Patent Document 9).
Then, in 1999, a resin bonded magnet with a (BH)max of 193 kJ/m3 is obtained in such a manner that an Nd—Fe(Co)—B ingot is heat-treated in hydrogen atmosphere such that: Nd2(Fe, Co)14B phase is hydrogenated (hydrogenation, Nd2(Fe, Co)14BHX); the phase is decomposed at 650 to 1000° C. (decomposition, NdH2+Fe+Fe2B); hydrogen is desorbed (desorption); and recombination is performed (recombination). Finally, HDDR Nd2Fe14B based magnetic materials are solidified with resin at 1 GPa (see Non-Patent Document 10).
In 2001, Mishima et al. reported Co-free d-HDDR Nd2Fe14B based magnetic materials (see Non-Patent Document 11), and N. Hamada et al. obtained a cubic anisotropic magnet (7 mm×7 mm×7 mm) with a density of 6.51 Mg/m3 and a (BH)max of 213 kJ/m3 in such a manner that d-HDDR Nd2Fe14B based magnetic materials with a (BH)max of 358 kJ/m3 are compressed together with resin at 0.9 GPa and at temperature of 150° C. in orientation magnetic field of 2.5 T (see Non-Patent Document 12).
<Patent Document>    <Patent Document 1> Patent Application No. Sho 62-196057
<Non-Patent Documents>    <Non-Patent Document 1> R. W. Lee, E. G Brewer, N. A. Schaffel, “PROCESSING OF NEODYMIUM-IRON-BORON MELT-SPUN RIBBONS TO FULLY DENSE MAGNETS” IEEE Trans. Magn., Vol. 21, 1985    <Non-Patent Document 2> G. X. Huang, W. M. Gao, S. F. Yu, “Application of Melt-spun Nd—Fe—B Bonded magnet to the Micromotor”, Proc. of the 11th International Rare-Earth Magnets and Their Applications, Pittsburgh, USA, pp. 583-594 (1990)<    <Non-Patent Document 3> B. H. Rabin, B. M. Ma, “Recent developments in NdFeB Powder”, 120th Topical Symposium of the Magnetics Society of Japan, pp. 23-30 (2001)<    <Non-Patent Document 4> S. Hirosawa, H. Kanekiyo, T. Miyoshi, K. Murakami, Y. Shigemoto, T. Nishiuchi, “Structure and Magnetic properties of Nd2Fe14B/FeXB-type nanocomposites prepared by Strip casting”, 9th Joint MMM/INTERMAG, CA (2004) FG-05    <Non-Patent Document 5> H. A. Davies, J. I. Betancourt R. and C. L. Harland, “Nanophase Pr and Nd/Pr-based Rare Earth-Iron-Boron Alloys”, Proc. of 16th Int. Workshop on Rare-Earth Magnets and Their Applications, Sendai, pp. 485-495 (2000)<    <Non-Patent Document 6> G. Tokunaga, “Magnetic Characteristic of Rare-Earth Bond Magnets, Magnetic Powder and Powder Metallurgy”, Vol. 35, pp. 3-7 (1988)<    <Non-Patent Document 7> T. Mukai, Y. Okazaki, H. Sakamoto, M. Fujikura and T. Inaguma, “Fully-dense Nd—Fe—B Magnets prepared from hot-rolled anisotropic powders”, Proc. 11th Int. Workshop on Rare-Earth Magnets and Their Applications, Pittsburgh, pp. 72-84 (1990)<    <Non-Patent Document 8> M. Doser, V. Panchanacthan, and R. K. Mishra, “Pulverizing anisotropic rapidly solidified Nd—Fe—B materials for bonded magnets”, J. Appl. Phys., Vol. 70, pp. 6603-6605 (1991)<    <Non-Patent Document 9> T. Iriyama, “Anisotropic bonded NdFeB magnets made from Hot-upset powders”, Polymer Bonded Magnet 2002, Chicago (2002)<    <Non-Patent Document 10> K. Morimoto, R. Nakayama, K. Mori, K. Igarashi, Y. Ishii, M. Itakura, N. Kuwano, K. Oki, “Anisotropic Nd2Fe14B-based Magnet powder with High remanence produced by Modified HDDR process”, IEEE. Tran. Magn., Vol. 35, pp. 3253-3255 (1999)<    <Non-Patent Document 11> C. Mishima, N, Hamada, H. Mitarai, and Y. Honkura, “Development of a Co-free NdFeB Anisotropic bonded magnet produced from the d-HDDR Processed powder”, IEEE. Trans. Magn., Vol, 37, pp. 2467-2470 (2001)    <Non-Patent Document 12> N. Hamada, C. Mishima, H. Mitarai and Y. Honkura, “Development of Nd—Fe—B Anisotropic Bonded Magnet with 27 MGOe” IEEE. Trans. Magn., Vol. 39, pp. 2953-2955 (2003)<    <Non-Patent Document 13> Z. Chena, Y. Q. Wub, M. J. Kramerb, B. R. Smith, B. M. Ma, M. Q. Huang, “A study on the role of Nb in melt-spun nanocrystalline Nd—Fe—B magnets', J., Magnetism and Magn., Mater., 268. pp. 105-113 (2004)”
Considering resin bonded magnets where the above descried anisotropic rare earth-iron based magnetic materials are solidified with resin at 0.9 GPa, for example, it is possible to gain the magnetic property of a (BH)max that is more than as twice as an isotropic resin bonded magnet with 80 kJ/m3. However, for adapting the anisotropic resin bonded magnets to rotary machines, it would be necessary to satisfy magnetic stability such as demagnetizing strength against irreversible demagnetization or demagnetizing fields.
Here, compared to the grain size 15-20 nm of an isotropic Nd2Fe14B based magnetic material obtained through a rapid-solidified thin strip (for example, see Non-Patent Document 13), an anisotropic Nd2Fe14B based magnetic material obtained through either the milling of hot-worked bulks or HDDR treatments has the grain size of 200 to 500 nm which is the texture of a Nd2Fe14B crystal that is one digit larger than the isotropic Nd2Fe14B based magnetic materials.
In case that the grain size of Nd2Fe14B is, for example, 15 to 20 nm, magnetic properties (including magnetic stability), such as remanence Mrp based on remanence enhancement effects or temperature coefficient βp%/° C. of coercivity HcJp, are improved. In addition, the magnetic properties such as HcJp or (BH)maxp of the magnetic materials would be not prominently deteriorated even if the particle size becomes lessened approximately to, for example, 40 μm.
That is, in case that the grain size of Nd2Fe14B is, for example, 15 to 20 nm, at the stage where the materials are compressed with resin, for example, at 0.8 to 1.0 GPa so as to obtain resin bonded magnets in a specific form, it would be inevitable that the surface of the magnetic material are damaged or fractured. However, the magnetic property deterioration of the magnetic materials is within a range that can be actually ignored.
Here, when considering Nd2Fe14B based magnetic materials where hot-worked bulks with a Nd2Fe14B grain size of 200 to 500 nm are milled, or anisotropic resin bonded magnets where HDDR-Nd2Fe14B based magnetic materials are solidified with resin at 0.8 to 1.0 GPa, occurrence of newly created surfaces or microcracks would be inevitable due to the damage or breakage of the surface of magnetic materials through densification. Accordingly, Nd2Fe14B crystals formed on the most outer surface of the magnetic materials are oxidized so as to cause texture evolution, whereby magnetic properties based on HcJp, (BH)maxp, etc. may be deteriorated. The treatment deterioration of the magnetic properties of the anisotropic Nd2Fe14B based magnetic materials is obvious compared to the isotropic Nd2Fe14B based magnetic materials. Thus, in order to suppress the deterioration of the magnetic properties occurring when the anisotropic Nd2Fe14B based magnetic materials are densified, it would be necessary to reduce or modify pressures toward the magnetic materials through the densification.
On the other hand, considering magnetic materials that have a nucleation-typed coercive generation mechanism which is typical in SmCoS base or Sm2Fe17N3 base, they generally need a particle size of 10 μm or less. As to resin bonded magnets where these magnetic materials with such a small particle size are compressed with resin, it would be difficult to make their densities to be 5 Mg/m3 or more (relative density: 65%). Accordingly, these resin bonded magnets are generally used as an injection-molded resin bonded magnet. Therefore, compared to an isotropic Nd2Fe14B based resin bonded magnet with a (BH)max of approximately 80 kJ/m3 where an isotropic Nd2Fe14B based magnetic materials are milled and solidified with resin at 0.8 to 1 GPa, the advantage of (BH)max is far behind, largely lowered than the (BH)max of an anisotropic Nd2Fe14B based resin bonded magnet.
It can be therefore said that these technical problems discussed hereinabove could be one of the factors which hampers an anisotropic rare earth-iron based resin bonded magnet from being applied to electromagnetic driving devices such as rotary machines although the anisotropic rare earth-iron based resin bonded magnet is regarded as the next generation type of the isotropic Nd2Fe14B based resin bonded magnet with a (BH)max of 80 kJ/m3.